1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and a developing device and a developing unit thereof, and, more particularly, to an image forming apparatus having a plurality of developing units to supply developers to an image carrier, and a developing device and a developing unit thereof.
2. Description of the Related Art
Image forming apparatuses to form an image on a printing medium according to an input signal. Examples of image forming apparatuses include printers, copiers, facsimiles, and devices combining functions thereof.
Of a variety of image forming apparatuses, a printing operation of an electro-photographic image forming apparatus is carried out in such a way that an electrostatic latent image is formed on a surface of a photosensitive member, which was charged with a predetermined electric potential, via scanning of light, and developer is supplied to the electrostatic latent image, forming a visible developer image. The developer image, formed on the photosensitive member, is transferred to a printing medium directly or by way of an intermediate transfer member. The transferred image present on the printing medium is fixed to the printing medium via a fixing process.
In the above-described printing operation, note that the developer image, formed on the photosensitive member or the intermediate transfer member, partially remains on the photosensitive member or the intermediate transfer member, rather than being completely transferred to the intermediate transfer member or the printing medium. The resulting remaining waste developer is collected by a cleaning device and is returned into a waste developer storage container.
Generally, the cleaning device includes a cleaning blade to come into contact with a surface of an image carrier, such as the photosensitive member or the intermediate transfer member, by a predetermined pressure. One end of the cleaning blade comes into frictional contact with the surface of the image carrier, to scrape the developer remaining on the surface of the image carrier.
An appropriate amount of developer remaining on the surface of the image carrier does not present a problem. However, where substantially no developer remains on the surface of the image carrier for a warm-up operation of the image forming apparatus, or in the case where only an inappropriately small amount of developer remains on the surface of the image carrier as a result of using a high transfer efficiency printing medium, a large frictional force is exerted between the cleaning blade and the image carrier, overturning the cleaning blade.
Further, even if the cleaning blade is not overturned, an excessively increased frictional force between the image carrier and the cleaning blade results in damage to the cleaning blade or loud frictional noise.
To solve the above-described problems, for example, there has been conventionally proposed a method wherein an image for lubrication is formed on the image carrier during a warm-up period or after printing a page prior to printing a subsequent page in a successive printing operation, so as to reduce friction between the image carrier and the cleaning blade.
However, when the above-described method is applied to a color image forming apparatus having a plurality of developing units, the following problems may occur.
Generally, a color image forming apparatus includes four developing units corresponding to respective colors. Any one of the four developing units is devised to supply a developer to an image carrier not only during a printing operation forming an image on a printing medium, but also during a lubricating operation forming an image for lubrication on the image carrier. The developing unit for lubrication consumes more developer than the other developing units, and thus, naturally has a shorter exchange cycle than other developing units. This forces a user to inconveniently exchange a specific developing unit frequently, and moreover, may cause users, not having advanced knowledge as to an operation of the image forming apparatus, to have a negative opinion thereof.
Another problem is that the color image forming apparatus cannot resume a printing operation until the image for lubrication formed on the image carrier passes all of the developing units downstream thereof. This may result in degradation of printing speed in the case of a successive printing operation.
Furthermore, the image for lubrication formed on the image carrier tends to be deflected toward the developing units arranged downstream thereof while passing through the developing units. This causes contamination of the developing units, resulting in degradation of image quality during a subsequent printing operation.
Meanwhile, there is a problem in that developer supplied to the image carrier for image formation is not completely attached to the image carrier, and partially falls or is scattered, contaminating an interior of the developing unit. In particular, if the residual developer enters a light window of a light scanning device, the developer may cause degradation of image quality during a printing operation.